Various types of exposure systems are currently in use for imprinting micro-patterns onto the surfaces of substrates such as semiconductor wafers. A typical exposure system includes an illumination source, a first stage apparatus that holds and positions a pattern master (e.g., a reticle), a second stage apparatus (downstream of the first stage apparatus) that holds and positions the substrate, a projection-optical system situated between the first and second stage apparatus, and a control subsystem connected to and exercising operational control over these apparatus and subsystems. Since the sizes of the pattern elements are very small (now in the several tens of nanometers), the first and second stage apparatus must be capable, as controlled by the control subsystem, of achieving extremely accurate and precise positioning of the stage apparatus and projection-exposure system relative to each other so as to achieve corresponding accuracy of exposure.
Substantially all exposure systems currently in use employ various sensors, detectors, and other measurement devices for determining and monitoring the accuracy and precision of stage position and of many other operations performed by the exposure system. An example use of sensors and detectors is in devices for performing auto-focus of the imaging performed by the exposure system. Auto-focus involves accurate and precise placement of the reticle and wafer relative to the exposure-optical system so that exposures made on the wafer have a specified imaging resolution.
For use in auto-focus devices, fluid gauges have been considered for use, either alone or in cooperation with other devices such as slit-projection sensors as described in U.S. Pat. No. 4,650,983. A first conventional example of such a device, called an “air gauge,” is discussed in U.S. Pat. No. 4,953,388, in which the device is configured as a pneumatic bridge. The gauge includes an air source, from which an air conduit is split to form a measurement arm and a reference arm. Each arm has a respective “probe” from which air is discharged onto a surface. For the measurement arm the surface is that of a workpiece. For the reference arm the surface is part of the gauge and is at a fixed distance from the probe. A mass-flow controller is connected between the arms to detect changes in air flow between the two arms resulting from a change in gap distance from the measurement probe to the workpiece. U.S. Pat. No. 5,540,082 discusses other conventional air gauges used for determining and monitoring position of a workpiece. Rather than using a mass-flow controller for determining differential flow of air to the two arms of the gauge, a differential pressure sensor is used. Also, the reference probe has been replaced with a controlled air-bleed device. Changes in gap distance are thus inferred from changes in the mass flow or pressure difference between the measurement and reference arms. These air gauges are sensitive to variations in ambient pressure and/or supply pressure, allowing these variations to introduce errors into wafer-height measurements. The magnitude of these errors tends to increase with corresponding increases in the dynamic pressure range in which the gauges operate.
Thus, conventional fluid gauges are sensitive to external influences that can degrade their operational accuracy and precision. Fluid gauges are also limited by available sensor technology. Sensors with larger dynamic range often suffer from lower resolution and increased electrical noise. Therefore, air gauges have been developed that employ multiple reference arms that can be selectively used with a measurement arm to increase the dynamic range of the fluid gauge. In this regard, reference is made to U.S. Patent Publication No. 2011/0157576, which is incorporated herein by reference to the maximum extent allowable by law. A fluid gauge as discussed in the '576 reference includes a gauge body having a measurement conduit and multiple reference conduits. The reference conduits have respective probes that are separated from respective reference surfaces by different respective reference gaps, across which the fluid is discharged. Alternatively, the reference probes can be replaced by controlled fluid-bleed devices. The measurement conduit has an outlet that is separated from the workpiece surface by a distance that is subject to change and that is determined by the gauge. A control system determines the position of the workpiece from the respective pressure differences between the measurement conduit and the reference conduits. (These pressure differences are termed respective “differential pressures.”) The reference conduits and measurement conduit also include respective flow restrictors that are substantially identical. By utilizing multiple reference channels in parallel having different controlled fluid bleeds, the maximum magnitude of the differential pressure can be reduced compared to what otherwise would be obtained using one reference conduit, without limiting the dynamic range of the fluid gauge.
An example is shown in FIGS. 1A and 1B, wherein FIG. 1A is a schematic diagram of a fluid gauge having three reference “channels” and one measurement “channel.” Each reference channel has a respective gap and thus a respective measurement pressure (“reference 1,” “reference 2,” and “reference 3”). The reference channels are arranged in parallel. Thus, associated with the measurement channel are three respective differential pressures (“DP1,” “DP2,” and “DP3”) that correlate to the respective references 1, 2, and 3. FIG. 1B depicts simulation data for a 2-mm diameter probe or “bearing” operating at a supply pressure of 2×105 Pa and a nominal gap of 20 μm. The -⋄-⋄-⋄- plot is of differential pressure, and the -□-□-□- plot is of the estimated force (“load”) applied by the probe against the substrate in the gap (wafer height) range of −1.0×104 to 1.0×104 nm (denoted on the abscissa as −1.0E+04 to 1.0E+04 nm, respectively). Employing only one reference channel in this gauge would require that the reference arm cover a DP range of −3×104 to 5×104 Pa (denoted on the ordinate as −3.0E+04 to 5.0E+04, respectively, and corresponding to a pressure (load) range of 0.005 to 0.05 N). By utilizing multiple reference channels in parallel, each with a different respective gap or controlled-bleed setting, each differential pressure DP1, DP2, DP3, while utilized for determining height, covers approximately one-third the full dynamic pressure range. The specific values of DP1, DP2, DP3 are monitored by a controller (not shown). Meanwhile, gaps or controlled bleeds are adjusted so that the pressure range in which differential pressure can be measured is divided by approximately three. FIG. 1B shows example DP1, DP2, DP3 ranges of this example. Note that DP1, DP2, and DP3 do not overlap each other. Consequently, only one DP signal lying within the dynamic range of the sensor can be obtained at a given moment in time. The controller monitors the three sensors continuously and selects the sensor lying within its dynamic range to determine the height.
Fluid gauges such as those discussed in the '576 application are not immune to the adverse effects of noise produced by the differential pressure sensors. This noise degrades the accuracy and precision of the position measurements produced by the fluid gauge; i.e., the noise introduces “gap error.” Fluid gauges as discussed in the '576 application are also sensitive to variations in ambient pressure and/or variations in the supply pressure, which also contribute to gap error. As described below, gap error is substantially reduced by incorporating, in the fluid gauge, multiple reference channels with respective DP sensors, wherein the individual DP ranges covered by the reference DP sensors largely overlap each other. Also, by using a weighted average of the data produced by the DP sensors, a more accurate and precise height determination is made that is less sensitive to sensor noise than height measurements produced by conventional fluid gauges.